Add Marionette.deprecate

Resolves #1792.

We should talk about what we would like to deprecate in future. See: https://github.com/marionettejs/backbone.marionette/issues/1792#issuecomment-54090366
I think I would like that sort of structure, actually. It will enforce the usage of Marionette.deprecate: warning against things that are being removed that you can act on *now*. I can even see it accepting three arguments, the third being a URL to reference.

API:

##### Marionette.deprecate( oldFeature, newFeature, referenceUrl )

> Deprecation warning: ___ is scheduled to be removed in v3.0.0. Please use ____ instead. (______)

Example:

> Deprecation warning: Marionette.Whatever is scheduled to be removed in v3.0.0. Please use Marionette.NewThing instead. (http://www.marionettejs.com/whatever/)